Is glass really a liquid?

The evidence being that very old window panes are thicker at the bottom because they’ve flowed down over the centuries. Let’s dispatch this piece of misinformation for a start. Yes, many old windows are thicker at the bottom, but they were thicker at the bottom when they were installed. Even so, glass is a liquid. Sort of.

The main ingredient of glass is silicon dioxide, which we also know as silica, quartz, or plain old sand. Typically, silica is crystalline when solid, meaning that its constituent atoms are packed closely together in an extremely regular way, so regular that their microscopic arrangement determines the shape and properties of the whole crystal.

Liquids are not so regimented. The atoms in liquids are fairly close together, but they lack regular structure and are somewhat free to move about, which is why liquids can flow.

Liquid silica has a neat trick, though. It’s very thick near its melting point and this means that if it’s rapidly cooled it never gets the chance to crystallise. Instead it just contracts, getting thicker and thicker as it cools, but never losing the free and easy structure of a liquid. Eventually it becomes effectively rigid — you could say it still flows, but only on a time scale of millions of years. Silica isn’t the only substance that can do this. Another, more delicious, example is hard toffee.

Nevertheless, if glass is a liquid it’s a very strange one. No matter how much tonic you add, a glass of glass will never make a refreshing drink on a summer’s day. It sounds like another of those “facts” much beloved by truculent adolescents and other contrarians who use overly technical definitions to prove that tomatoes are not vegetables, strawberries are not berries and peanuts are not nuts.

Clearly glass is solid, in the everyday sense, yet there is no point during cooling where you can say it stops being a liquid. Everyday language fails us here. Call it a liquid if you like but you must admit that, as liquids go, glass a very solid one.

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